1. Field of the Invention
The present invention relates to the use of a high energy electron beam to chemically transform or destroy certain types of hazardous waste, and more particularly, to an electron window that promotes efficient transfer of a diverging high energy electron beam into a detoxification vessel.
2. Description of Related Art
Volatile organic compounds (VOCs) exist in the form of vapors or gasses that are emitted or vaporized from hazardous or toxic waste materials. Since these VOCs pose a significant health risk to individuals and to the environment, it is necessary to contain, extract and collect the hazardous materials so as to prevent spreading of the VOCs into the air and/or ground water. Once contained, the VOCs can be remediated by converting them into less hazardous materials that can be disposed of with substantially reduced risk.
One such remediation technique involves the injection of a high energy electron beam into a detoxification vessel containing the VOCs. Interaction between the electrons of the beam and the VOCs causes chemical transformation of the VOCs in three significant aspects, including: (1) direct de-chlorination resulting in inorganic chloride ions and reactive organic intermediates which are further degraded into non-reactive compounds; (2) production of organic and inorganic free radicals and ions which are reactive and whose reactions result in destruction of the target hazardous materials; and (3) formation of aqueous electrons (in the presence of water vapor) capable of reducing chemical bonds. An example of a toxic remediation device comprising an electron beam source coupled to a detoxification vessel is disclosed in U.S. Pat. No. 5,319,211.
In order to achieve a sufficient level of remediation within the detoxification vessel, it is desirable to provide an electron beam having relatively high energy (in excess of -160 kilovolts DC). Diverging electron beams are generally desirable since they tend to distribute the energy of the beam evenly across a wide region of the vessel. The electron beam source operates in a vacuum environment, and the diverging electrons of the beam must pass through an electron window that provides a barrier with the non-vacuum environment of the detoxification vessel. The electron window typically comprises a support grid having a plurality of holes extending therethrough, with a sheet of metal foil disposed on a downstream side of the support grid. Ideally, the electrons pass through the holes and the foil with minimal interception by the support grid itself, since such interceptions result in undesirable energy loss to the support grid.
In practice, however, it is difficult to efficiently transmit the diverging electrons of the beam through the electron window into the vessel. The holes of the support grid must be large enough to accommodate the diverging path of the electrons, but when the holes in the support grid are too large, the temperature differential (.DELTA.T) within the foil between the center of the hole and the edge of the hole becomes too high, and the foil loses structural and thermal integrity. Similarly, as the holes represent an increased percentage of the surface area of the support grid, the support grid becomes less able to withstand the force created by the vacuum pressure used to evacuate the electron beam source structure during its assembly. This vacuum force imposes significant mechanical stress on the support grid, and in some cases may cause deformation or bowing of the support structure. Such deformation further reduces the effective hole size relative to the diverging electron paths, thereby reducing electron transmission. At the same time, the thermal conduction path through the support grid decreases and therefore the AT in the support grid increases.
The temperature of the metal foil at the center of each hole equals the coolant temperature plus the .DELTA.T of the support grid plus the .DELTA.T in the foil, measured from the edge of the hole to the center of the hole. Titanium is a desirable material for the metal foil, due to its high strength, low atomic number for electron transmission (i.e., good electron transmissivity), high melting point, and high vacuum compatibility. It is necessary, however, to keep the foil temperature below 400.degree. C., otherwise the titanium foil transforms from an e crystalline form to a .beta. crystalline form. The .beta. form of titanium is more susceptible to oxidation, which leads to embrittlement and a higher likelihood of rupture (i.e., loss of vacuum within the electron beam source structure). This problem may be further exacerbated by the high operating voltage of the device, since the high current density (W/cm.sup.2) through a large hole size may result in destructively high .DELTA.T levels in the foil.
Thus, a critical need exists for an efficient electron window to provide a vacuum barrier between a high power electron beam source and an associated detoxification vessel for remediation of VOCs from hazardous or toxic waste. Such an electron window should be able to efficiently conduct a diverging electron beam into the vessel without compromising either its thermal or structural capability.